Hormone Receptors of the Baboon Cardiovascular System: Biochemical Characterization of Aortic and Myocardial Cytoplasmic Progesterone Receptors

We used the synthetic progestin R5020 (17α,21-dimethyl-19-norpregna-4,9-diene- 3,20-dione) to characterize cytoplasmic progesterone receptors in baboon(Pupio sp.) aorta and myocardium. The relative ability of selected steroids to inhibit binding of radiolabeled R5020 to aortic cytoplasmic progesterone receptors was radioinert R5020, 1.0; progesterone, 0.31; triamcino- lone acetonide, 0.06; testosterone, 0.002; estradiol-17β, 0.01; and cortisol, 0.001. The relative ability of these same steroids to inhibit binding of radiolabeled R5020 to myocardial cytoplasmic progesterone receptors was, respectively, 1.0, 0.58, 0.21, 0.01, 0.01, and 0.001. Both aortic and myocardial cytoplasmic progesterone receptors migrated as macromolecules with a sedimentation coefficient of 8–9S on low ionic strength linear sucrose gradients. Cytoplasmic binding of R5020 was inactivated by incubation at 37°C. Saturation analysis at 2°C showed aorta and myocardium, respectively, contained 41.6 ± 16.1 (mean ± SD) and 14.0 ± 2.8 fmol R5020 binding sites/mg cytosol protein. The dissociation constant for R5020 was 2.8 ± 1.2 nm, aorta, and 2.0 ± 1.1 nM, myocardium. The presence of progesterone receptors in baboon cardiovascular tissues suggests that progestins may directly influence cardiovascular tissue function.